The production of alkene oxides, or epoxides, particularly ethylene oxide by the direct oxidation of the corresponding alkene in the presence of a silver-containing catalyst has been known for many years. For example, the basic process was described by Lefort in U.S. Pat. No. 1,998,878 and by Van Peski in U.S. Pat. No. 2,040,782. The basic reaction proceeds, as illustrated for ethylene, according to the equation: ##STR1## and production of an unwanted by-product according to the reaction: EQU CH.sub.2 .dbd.CH.sub.2 +3 O.sub.2 .fwdarw.2 CO.sub.2 +2 H.sub.2 O (II)
or by further oxidation of the epoxide.
In the years between the Van Peski patent and the present inventions, research efforts have been directed to improving both the activity and longevity or useful life of the catalyst and the efficiency of the overall catalytic reaction. As is indicated by reactions I and II, the oxidation of an alkene may produce either the alkene oxide (I) sought in the process or the by-products CO.sub.2 and H.sub.2 O.
Several terms are commonly used to describe some of the parameters of the catalytic system. For instance, "conversion" has been defined as the percentage of alkene fed to the reactor which undergoes reaction. The "efficiency" or, as it is sometimes called, the "selectivity" of the overall process is an indication of the proportion, usually represented by a percentage, of the converted material or product which is alkene oxide. The commercial success of a reaction system depends in large measure on the efficiency of the system. At present, maximum efficiencies in commercial production of ethylene oxide by epoxidation are in the low 80s, e.g., 80 or 81 percent. Even a very small increase in efficiency will provide substantial cost benefits in large-scale operation. For example, taking 100,000 metric tons as a typical yearly yield for a conventional ethylene oxide plant and assuming 80 percent conversion, an increase in efficiency of from 80 to 84 percent, all other things being equal, would result in a savings of 3790 metric tons of ethylene per year. In addition, the heat of reaction for reaction II (formation of carbon dioxide) is much greater than that of reaction I (formation of ethylene oxide) so heat-removal problems are more burdensome as the efficiency decreases. Furthermore, as the efficiency decreases, there is the potential for a greater amount of impurities to be present in the reactor effluent which can complicate separation of the desired alkene oxide product. It would be desirable, therefore, to develop a process for the epoxidation of alkene in which the efficiency is greater than that obtained in conventional commercial processes, e.g., with ethylene, efficiencies of 84 percent or greater, while maintaining other performance characteristics, particularly the activity, as described below, in a satisfactory range.
The product of the efficiency and the conversion is equal to the yield, or the percentage of the alkene fed that is converted into the corresponding oxide.
The "activity" of the catalyst is a term used to indicate the amount of alkene oxide contained in the outlet stream of the reactor relative to that in the inlet stream. Activity is generally expressed in terms of pounds of alkene oxide produced per cubic foot of catalyst per hour at specified reaction conditions and rate of feed. The activity may also be stated in terms of the amount of ethylene oxide in the outlet stream or the difference between the ethylene oxide content of the inlet and outlet streams.
If the activity of a reaction system is low, then, all other things being equal, the commercial value of that system will be low. The lower the activity of a reaction system, the less product produced in a unit time for a given feed rate, reactor temperature, catalyst, surface area, etcetera. A low activity can render even a high efficiency process commercially impractical. For production of ethylene oxide, an activity below 4 pounds of ethylene oxide per hour per cubic foot of catalyst is unacceptable for commercial practice. The activity is preferably greater than 8 pounds, and in some instances an activity greater than 11 pounds of alkene oxide per hour per cubic foot of catalyst is desired.
In some instances, activity is measured over a period of time in terms of the amount of alkene oxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant amount of alkene oxide. Plots of such measurements yield "aging rates" which reflect the stability or useful life of the catalyst. The useful life of a reaction system is the length of time that reactants can be passed through the reaction system during which acceptable activity is observed. The area under a plot of activity versus time is equal to the number of pounds of alkene oxide produced during the useful life of the catalyst per cubic foot of catalyst. The greater the area under such a plot, the more valuable the process is since regeneration or replacement of the catalyst involves a number of expenses, sometimes referred to as turnaround costs. The rate at which activity decreases, i.e., the rate of deactivation at a given point in time, can be represented by the slope of the activity plot, i.e., the derivative of activity with respect to time: EQU deactivation=d[activity]/dt.
The average rate of deactivation over a period of time can be represented then by the change in activity divided by the time period: EQU average deactivation=.DELTA. activity/.DELTA. t.
At some point, the activity decreases to an unacceptable level, for example, the temperature required to maintain the activity of the system becomes unacceptably high or the rate of production becomes unacceptably low. At this point, the catalyst must either be regenerated or replaced. Some of these definitions may be represented as set out below: ##EQU1##
Typically, in commercial production, since the outlet or effluent stream emanating from the reactor may contain substantial amounts of unreacted alkene, the effluent stream is recycled and combined with the feedstream after removal of at least a portion of the alkene oxide. Generally, as the activity of a catalyst decreases with time, in order to obtain the same ultimate yield of epoxide product, the effluent stream must either be recycled a greater number of times or the temperature within the reactor must be raised to increase the activity of the catalyst. The former approach to increasing the yield of product requires additional energy expenditures and the latter, which is most frequently used, causes faster catalyst deterioration.
As used herein, an activity-reducing compound refers to a compound which, when present in an activity-reducing amount, causes a reduction in activity, some or all of which activity may subsequently be regained by returning to a situation in which the concentration of the compound is below the minimum activity-reducing amount. The minimum activity-reducing amount varies depending on the particular system, the feedstream and the activity-reducing compound.
Conversely, deactivation, as used herein, refers to a permanent loss of activity, i.e., a decrease in activity which cannot be recovered. As noted above, activity can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular activity is representative of deactivation. Furthermore, catalysts tend to deactivate more rapidly when reaction is carried out at higher temperatures.
In contrast to problems associated with low or decreasing catalyst activities, less than satisfactory efficiencies result in loss of starting material, the alkene, as the unwanted product CO.sub.2. Ultimately, this also increases product costs.
To be considered satisfactory, a catalyst must not only have a sufficient activity and the catalytic system provide an acceptable efficiency, but the catalyst must also demonstrate a minimum useful life or stability. When a catalyst is spent, typically the reactor must be shut down and partially dismantled to remove the spent catalyst. This results in losses in time and productivity. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of regeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in time to treat the spent catalyst and, at worst, requires replacement of the catalyst with the associated costs.
Since even small improvements in activity, efficiency or useful life may have significance in large scale commercial production, such improvements have been the object of a great deal of research in the direct epoxidation of alkenes. The focus of attempts to improve performance, such as the activity and useful life of the catalyst and the efficiency of the system, has included such areas as feedstream additives or removal of components therefrom; methods of preparation of the catalyst; deposition or impregnation of a particular type or form of silver; composition, formation, physical properties and morphology of the support; additives deposited on or impregnated in the support; shape of support aggregates used in the reactor; and various types of reactors and bed designs, such as stationary and fluidized beds.
Early work on the silver-catalyzed direct oxidation of alkenes to alkene oxides in many instances resulted in improvements in activity and particularly the selectively of the system, in many cases the efficiency increasing by several percent. However, recent modifications in such systems have resulted in only small incremental improvements in efficiency. In terms of operating costs, even fractions of a percent improvement in efficiency can translate into large savings in production. Accordingly, current research is still being directed to improvements in the activity and useful life of the catalyst and selectivity of the system.
Although a vast number of elements and compounds are known to have effective catalytic properties in various reactions, many have at least one shortcoming, such as very high cost and/or limited availability, thermal instability in the temperature range in which the reaction is to be conducted, low mechanical strength, small surface area per unit of volume, susceptibility to poisoning, short useful lifetime, etcetera. Such undesirable characteristics make such substances of limited utility as catalysts. Some of these shortcomings, however, may be overcome and in some instances the effectiveness of the catalyst may be improved by applying the substance to a carrier or support.
New support materials are continuously being tried. However, many of those which were employed in the early development of the silver-bearing catalysts are, with some modifications, still being used. Materials which have found most widespread use are typically inorganic and generally are of a mineral nature. Such materials commonly include alumina, fire brick, clay, bauxite, bentonite, kieselguhr, carbon, silicates, silica, silicon carbide, zirconia, diatomaceous earth, and pumice.
In addition to the physical strength of the support materials, other physical properties, such as surface area, pore volume, pore dimensions, and particle size have drawn considerable attention. These properties have been examined with great scrutiny when evidence indicated that there was a correlation between the size of silver particles and the efficiency of the overall system or useful life of the catalyst. Some materials are also preferred for their chemical properties, i.e., their "inertness" or "promoting" properties.
The support serves a number of functions in a heterogeneous catalytic system. Ease of handling is facilitated by a support which generally takes the form of discrete particles or aggregates of varying shape or size which, depending on usage, have a major dimension of about 1 millimeter to about 20 millimeters. Thus it is not necessary for the catalyst to form a permanent or semi-permanent part of the reactor.
The support, however, serves primarily to increase the surface area of the "active" component of the catalyst, silver, which is important in that most epoxidation occurs at the silver surface-fluid interface. Many of the substances commonly employed as catalyst supports not only have the usual external surface, which provides a varying surface area, depending on the shape of the support bodies and the packing of the bodies, but are also of a porous nature and, therefore, have a large internal surface which contributes to the overall surface area of the supported catalyst. Such support materials provide a greater capacity for sorbing not only the catalyst material during catalyst preparation, when the support is impregnated with a solution containing the catalyst component(s) in soluble form, but also a greater capacity for the flow of the fluid reactants within the catalyst during the reaction for which the catalyst is intended. The support also improves performance by lowering the pressure drop through the reactor and by facilitating heat and mass transfer.
Among the large variety of substances employed in the past as supports for catalytic materials, alumina has exhibited superiority in many respects as a catalyst support material. In addition to the low cost of the material, alumina has good thermal stability and some forms have a relatively large surface area.
Alumina, in its various forms, particularly alpha-alumina, has been preferred as a support material for silver-containing catalysts in the preparation of alkene oxides. Numerous variations of surface area, pore dimensions, pore volume and particle size have been suggested as providing the ideal physical property or combination of properties for improving efficiency, activity or useful life of the catalyst.
Holler (U.S. Pat. No. 3,908,002) discloses an alpha-alumina, useful as a catalyst support for reactions conducted at temperatures below 800 degrees C., such as oxidation reactions of hydrocarbons to oxyhydrocarbons. The support, having a surface area reported to be at least about 40 m.sup.2 /g, is produced by thermally decomposing a porous aluminum ion chain-bridged, polymeric carboxylate. Indicating that a large surface area in a carrier may be detrimental to its efficient operation and catalyst activity, Belon (U.S. Pat. No. 3,172,866) describes a method of producing a macroporous catalyst carrier which may be used in the catalytic production of ethylene oxide having pore diameters of between 0.1 and 8.0 microns and a specific surface area between a few square meters and one square decimeter per gram. The support is prepared by heating a mixture of active and calcined aluminum oxides and a small amount of boron oxide at temperatures of between about 1,600 and 1,800 degrees C. Waterman (U.S. Pat. No. 2,901,441) describes a process for preparing highly active and selective catalysts for the oxidation of olefins to olefin oxides on a support having an average porosity of at least 35 percent. The method involves washing an alpha-alumina or silicon carbide support having an average porosity of between 35 and 65 percent with an aqueous solution of lactic acid, washing with water until neutral, and then impregnating the support with an aqueous solution of silver lactate. The impregnated support is thereafter heat-treated to deposit elemental silver. A silver-supported catalyst for the vapor phase oxidation of ethylene to ethylene oxide, exhibiting improved production of ethylene oxide and catalyst longevity, is described by Brown et al (U.S. Pat. No. 3,725,307). The catalyst is disclosed as being formed from support particles having an average pore diameter of at least 10 microns up to, preferably, 70 microns and a surface area of less than about 1 m.sup.2 /g. The selectivities reported do not range above about 73 percent. The support is preferably composed of silica-alumina. A silver-supported catalyst which includes a support of alpha-alumina, silicon carbide, fused aluminum oxide, or mixtures of alumina and silica was asserted by DeMaio (U.S. Pat. No. 3,664,970) to eliminate the need for halogenated inhibitors in the oxidation of ethylene to ethylene oxide. The support is composed of particles having a minimum apparent porosity of about 30 percent and wherein at least 90 percent of the pores have diameters in the range of 1 to 30 microns, the average of the diameters being in the range of 4 to 10 microns. Wattimena (U.S. Pat. No. 3,563,914) discloses silver catalysts using aluminum oxide supports having pore volumes between 15 and 30 ml/g and surface areas below about 10 m.sup.2 /g.
Hayden et al (U.K. patent application 2,014,133) disclose a silver catalyst employing a support having a specific surface area in the range of 0.05 to 10 m.sup.2 /g, an apparent porosity of at least 20 percent, and mean pore diameters of 0.1 to 20 microns, the pore size distribution being bimodal, in which the smaller pores preferably account for at least 70 percent of the total pore volume. Alpha-alumina supports are described by Rashkin (U.K. patent application 2,122,913A) having a "relatively low surface area" of less than 30 m.sup.2 /g. Mitsuhata et al (Japanese Published patent application 56-089843) and Mitsuhata et al (U.S. Pat. No. 4,368,144) describe supported silver catalysts in which the support is formed from alpha-alumina having a specific surface area of 0.5 to 5 m.sup.2 /g. Watanabe et al (Japanese Published patent application 56-105750) employ a similar catalyst support having a surface area of 1 to 5 m.sup.2 /g. Hayden et al (U.S. Pat. No. 4,007,135) describe silver-containing catalysts in which the porous heat-resisting support has a specific surface area in the range of 0.04 to 10 m.sup.2 /g, an apparent porosity of at least 20 percent, and a median pore diameter of 0.3 to 15 microns. Mitsuhata et al (U.S. Pat. No. 4,248,740) describe the use of high alpha-alumina content supports having a specific surface area of not more than 10 m.sup.2 /g, an apparent porosity of 40 to 60 percent by volume, and a pore volume of 0.1 to 0.5 cc/g. Armstrong et al (U.S. Pat. No. 4,342,667) disclose a supported silver catalyst, useful in the oxidation of ethylene to ethylene oxide, in which the support has a surface area of 0.02 to 2 m.sup.2 /g, an average pore diameter of 0.5 to 50 microns and an average pore volume of 0.2 to 0.5 cc/g.
There has also been some interest in the purity of supports employed, both as to composition and phase. Examples of high purity alumina include U.S. Pat. No. 2,901,441 which uses alpha-alumina having a purity of about 99.5 percent as a support for catalysts used to oxidize olefins to olefin oxides. An ethylene oxidation catalyst is disclosed in German patent Publication DE 2,933,950 which attains a long catalyst life without a loss in activity or selectivity by using an alpha-alumina support having less than 0.001 weight percent of alkali-soluble silicon compounds. The catalyst is prepared by boiling commercial quality alpha-alumina with 1 weight percent sodium hydroxide solution and washing to a pH value of 8. If desired, the silicon compound concentration may be reduced below 1 part per million (ppm) by washing further with 1 weight percent HF. U.K. patent application 2,122,913A describes supported silver catalysts in which the support is composed of silica, alumina or mixtures thereof, one example of which is an alumina having a purity of 99.3 percent by weight. The silver-supported catalyst described in Japanese Published patent application 56-089843 employs an alpha-alumina carrier having a sodium content of less than 0.07 weight percent. Japanese Published patent application 56-105750 describes the use of an alpha-alumina support in conjunction with a silver catalyst for producing ethylene oxide, which support has a sodium content less than 0.07 weight percent. A silver catalyst including an alpha-alumina carrier having a sodium content of not more than 0.07 percent is described by Mitsuhata et al (U.S. Pat. No. 4,368,144). The support also has a surface area within the range of 0.5 to 5 m.sup.2 /g, an apparent porosity of 25 to 60 percent, a specific pore volume of 0.2 to 0.5 cc/g, and a particle diameter within the range of 3 to 20 mm. An alpha-alumina support having a purity of 98+weight percent, for use with silver in the catalytic oxidation of ethylene, is described by Warner et al in U.S. Pat. No. 4,455,392. The patent additionally discloses that the carrier is generally a conventional microporous support with surface areas of less than 10 m.sup.2 /g, pore volumes ranging from about 0.15 to 0.8 cc/g, and pore diameters of about 0.1 to 100 microns.
In addition to compositional purity, both phase purity and morphology of the support have been areas in which improvements in efficiency, selectivity or stability of the catalyst have been sought. Examples include U.S. Pat. No. 2,901,441 in which aluminum oxide is substantially completely converted to the alpha form of alumina by heating aluminum oxide to a temperature of about 1,500 to 2,050 degrees C. Weiss (U.S. Pat. No. 2,209,908) and Carter (U.S. Pat. No. 2,294,383) describe the use of "Tabular Corundum" as a catalyst support for metallic oxides, such as those oxides of metals selected from the fifth and sixth group of the periodic system, for example, vanadium, molybdenum, uranium, etcetera, in the oxidation of various organic materials to maleic acid and maleic anhydride and silver for the catalytic oxidation of ethylene to ethylene oxide, respectively. Weiss indicates that Tabular Corundum, which is almost entirely aluminum oxide and has the alpha-corundum crystalline form of aluminum oxide, may be formed by mixing aluminum oxide with one or more of several compounds, such as sodium oxide and chromic oxide, and heating the mixture to a temperature in the range of about 800 to about 1,800 degrees C. Tabular Corundum is further described as having impurities present in only small quantities, the material also includes "readily bonded surfaces and consisting essentially of interlocked corundum crystals in tabular form, having the contained impurities disseminated in minute globules throughout the crystalline alumina". Brengle et al (U.S. Pat. No. 2,709,173) also employ Tabular Corundum as a support in one of their examples.
U.S. Pat. Nos. 4,039,481 and 4,136,063 to Kimura et al disclose a catalyst carrier and a method for making same, the catalyst being the type used in catalytic converters in automobile exhaust systems. Specifically, the catalysts have a surface layer containing alpha-phase alumina and an inner portion consisting essentially of alumina of a phase other than that of the alpha phase. The pores in the alpha-alumina surface layer are larger than those in the inner portion of the catalyst body. A method of preparing the phase gradient support particles is described which provides for treating the surface of the alumina to a depth of about 400 microns with a transition element, particularly iron, and thereafter firing the carrier particles.
Weber et al (U.S. Pat. No. 4,379,134) describe high purity alpha-alumina bodies, at least 85 percent of the pore volume of the bodies having pores with a diameter of from 10,000 to 200,000 Angstroms. The high purity alpha-alumina bodies are prepared by peptizing boehmite in an acidic aqueous, fluoride anion-containing mixture. An extrudable mixture is formed thereby which is extruded and shaped into formed bodies which are thereafter dried at 100 to 300 degrees C., calcined at a temperature of from 400 to 700 degrees C. to convert the alumina to the gamma phase, and subsequently calcined further at a temperature of from 1,200 to 1,700 degrees C. to convert the gamma phase to alpha-alumina phase.
A method of producing granulated porous corundum having a homogeneous porous structure with a total pore volume of 0.3 to 1.0 cm.sup.3 /g and a predominant pore size of 5,000 to 30,000 A is described by Boreskov et al (U.S. Pat. No. 3,950,507). The method of preparing the alpha-alumina includes treating active alumina or aluminum hydroxide having a porous structure to a first heat treatment in which the temperature is increased from 20 to 700 degrees C., a second heat treatment in the range of from 700 to 1,000 degrees C., and a third treatment in the range of from 1,000 to 1,400 degrees C. Each of the heat treatments is for a period of at least one-half hour, the first heat treatment being conducted in an atmosphere of hydrogen fluoride in which the alumina absorbs the hydrogen fluoride and the second heat treatment desorbs the hydrogen fluoride. The patent also describes a similar procedure employing stationary thermal conditions in which the granules of alumina or aluminum hydroxide are impregnated with other fluorine-containing compounds prior to the first thermal treatment. The recommended starting materials used to form alpha-alumina include granulated pseudo-boehmite, boehmite or bayerite as the granulated aluminum hydroxide and granulated alpha-, eta-, or theta-alumina as the active alumina.
Although alpha-alumina has been considered by most to be the preferred alumina support material, Smith et al (U.S. Pat. No. 2,422,172) have suggested that beta-aluminas are more desirable than the alpha phase as a support material for catalysts, particularly those used in catalytic conversion processes such as dehydrogenation and hydroforming.
In seeking the ideal support material, there has been some departure from the commonly employed substances. For example, some use has been made of alkali metal and alkaline earth metal carbonates, both as the sole support material and in combination with other materials as the carrier for processes such as direct oxidation of alkenes to epoxides.
A number of supported silver-containing catalysts have been employed for epoxidation of alkenes in which the carrier includes, sometimes labelled as a promoter, a carbonate of a metal, generally an alkali metal or alkaline earth metal. Some examples of the use of one or more alkali and/or alkaline earth carbonates may be found in U.S. Pat. Nos. 2,424,084, 2,424,086, 2,615,900, 2,713,586, 3,121,099, 3,258,433, 3,563,913, 3,563,914, 3,585,217, 4,007,135, 4,033,903, 4,039,561, 4,066,575, 4,094,889, 4,123,385, 4,125,480, 4,168,247, 4,186,106, 4,226,782, 4,229,321, 4,324,699, European patent Publications 0,003,642 and 0,011,356, Japanese patents 41-11847 and 57-107242, U.K. patents 590,479, 1,571,123 and 2,014,133A, and Murray, "A Study Of The Oxidation Of Ethylene To Ethylene Oxide On A Silver Catalyst", Australian Journal of Scientific Research, Volume 3A, Pages 433-499 (1950). In addition, U.S. Pat. No. 3,332,887 employs zinc and/or cadmium carbonates, Gelbstein, (DS 2,352,608) discloses the use of the latter carbonate and European patent Publication 0,003,642 mentions the use of molybdenum carbonate.
Several patents have described the use of fluorine-containing compounds to treat support materials, in some cases to provide a compositionally pure support, and in other cases as a fluxing agent to improve the phase purity of the support. Thus, U.K. Published patent specification 590,479 and U.S. Pat. No. 2,424,086 indicate that a more active catalyst is formed if the support material has undergone a preliminary treatment with a dilute solution of hydrofluoric acid prior to impregnation with silver. U.S. Pat. No. 4,379,134 teaches the preparation of high purity alpha-alumina bodies by peptizing boehmite alumina in an aqueous acidic mixture containing fluoride anions and water. German patent 2,933,950 teaches the reduction of silicon content by treatment with HF. U.S. Pat. No. 3,950,507 teaches the preparation of granulated porous corundum by a multiple step heat treatment in which initial steps may be carried out in an atmosphere of hydrogen fluoride. Hosoda et al (U.S. Pat. No. 3,144,416) suggest that a small amount of a halogen compound, sulfur compound, nitrogen compound, or phosphorous compound may be added either to the reaction gas or the catalyst to improve the selectivity of the catalyst.
The nature of the silver itself has also been examined and modified in attempts to improve the efficiency and stability of the catalyst. Cavitt (U.S. Pat. No. 4,229,321) teaches that a supported silver catalyst of improved selectivity and activity may be prepared by mechanically removing the outer surface or skin of the catalyst after the impregnated catalyst has been heated to evaporate volatile material and reduction of the silver salt to silver metal, thereby activating the catalyst.
Since the early work on the direct catalytic oxidation of ethylene to ethylene oxide, workers in the field have suggested that the addition of certain compounds to the gaseous feedstream or direct incorporation of metals or compounds in the catalyst could enhance or promote the production of ethylene oxide. Such metals or compounds have been known variously as "anti-catalysts", "promoters" and "inhibitors". These substances, which are not themselves considered catalysts, have been proposed by prior workers to contribute to the efficiency of the process by inhibiting the formation of carbon dioxide or promoting the production of ethylene oxide. The scientific literature is replete with examples of the use of alkali metals and alkaline earth metals and their cations to promote the efficiency of silver catalysts used in epoxidation reactions. For example, sodium, potassium and calcium were disclosed as being suitable promoters in U.S. Pat. No. 2,177,361. Numerous examples may be found in literature of preference for one or several metals or cations and exclusion of one or more metals or cations as promoters in silver catalysts.
Among those anions associated with the cation used in preparing silver-containing catalysts employed in direct epoxidation reactions that have been suggested as being suitable include carboxylates, for example, formate, acetate, malonate, oxalate, lactate, tartrate, and/or citrate, and inorganic salts, such as carbonates, bicarbonates, phosphates, nitrates, and/or nitrites, chlorides, iodides, bromates, and isopropoxides. However, although many examples may be found in the literature indicating that such compounds are suitable, numerous patents, such as U.S. Pat. Nos. 3,962,136; 4,012,425; 4,066,575; 4,207,210; and 4,471,071, suggest that no unusual effectiveness, particularly with regard to catalytic activity, is observed with any particular anion of an alkali metal promoter. U.S. Pat. Nos. 4,007,135; 4,094,889; 4,125,480; 4,226,782; 4,235,757; 4,324,699; 4,342,667; 4,356,312; 4,368,144; and 4,455,392 disclose that potassium nitrate may be added to the catalyst as a suitable promoting material. Potassium nitrate may also be formed in situ when a carrier material is treated with certain amines in the presence of potassium ions as, for instance, when silver is introduced to a carrier material in a silver-impregnating solution containing an amine and potassium ions, followed by roasting.
A number of compounds have been proposed in the literature as additives to the feedstream or reactants to improve the efficiency of the direct, silver-catalyzed oxidation of alkenes to alkene oxides. For example, Law and Chitwood (U.S. Pat. No. 2,194,602) disclose the use of a "repressant", i.e., anti-catalyst, such as ethylene dichloride, chlorine, sulfur chloride, sulfur trioxide, nitrogen dioxide, or other halogen-containing or acid-forming materials. Numerous additional anti-catalysts are presented by the same patentees in U.S. Pat. No. 2,279,469. The anti-catalysts, broadly listed in categories such as halogens and compounds containing halogen, hydrocarbons, compounds containing carbon, hydrogen and oxygen, compounds containing sulfur, and compounds containing nitrogen are represented and, in addition to those compounds already mentioned above, additional representative compounds include, as nitrogen-containing compounds, nitric oxide, ammonia, amines such as ethylenediamine, diphenylamine and analine, nitro compounds such as o-nitroanisole and o-nitrotoluene as organic oxygen-containing organic compounds, alcohols such as methyl, ethyl and isopropyl alcohols, ethers such as isopropyl and dibutyl ethers, as well as glycol ethers, ketones such as methyl ethyl ketone and acetone, as hydrocarbons such as benzene, and N-hexane; sulfur compounds such as sulfur dioxide, hydrogen sulfide and diethylsulfide; chlorine-containing compounds such as carbon tetrachloride, chlorobenzene and dichloroethyl ether. Berl (U.S. Pat. No. 2,270,780) lists a number of compounds as anti-detonating or anti-knock materials to control the oxidation of ethylene and propylene to their oxides. Disclosures of other feedstream additives used in the production of alkene oxides, particularly halogen compounds, may be found in U.S. Pat. Nos. 2,279,470; 2,799,687; 3,144,416; 4,007,135; 4,206,128; and 4,368,144. In addition, EPO patent 0,003,642 and U.K. patent application 2,014,133A disclose processes for the production of olefin oxides employing silver-containing catalysts in which a chlorine-containing reaction modifier and a nitrite or nitrite-forming substance are described. Rumanian patent 53,012, published Dec. 2, 1971, discloses a direct, silver-catalyzed direct epoxidation procedure which employs oxides of nitrogen in the feedstream. U.K. patent 524,007 includes ethylene dichloride or nitrogen dioxide in the feedstream of a silver-catalyzed epoxidation procedure.
Although much of the art discussed above has resulted in improvements in the efficiency, activity or stability of the catalytic system, many of the improvements have individually been rather slight. In some of the catalytic systems, gains in one of these performance parameters have been frequently offset by losses in another; that is, enhancement of one index of performance has been accompanied by a deleterious effect on another of the indices. For example, if a reaction system is designed which has a very short useful life, the system may be commercially impractical even though the efficiency and initial activity of the catalyst are outstanding. Accordingly, a system that provides an increase in the efficiency of the overall catalytic reaction system, while only minimally affecting the activity and useful life of the catalyst, or perhaps increasing one of these performance indices, would be particularly beneficial.
The presence of leachable sodium in a silver catalyst employed for epoxidation tends, in some instances, to improve the efficiency of the system under epoxidation conditions generally used. In the presence of CO.sub.2 and certain efficiency-enhancing compounds, however, sodium exhibits deactivating and effective life-shortening effects on epoxidation catalysts and systems. The loss of stability or life-shortening effect may be so marked, particularly when the sodium is added as a promoter, that any gains in efficiency provided by the sodium are lost several days after the catalyst is placed in service. In many commercially used epoxidation reactors the effluent stream contains relatively high proportions of unreacted alkene. To diminish losses of the alkene, the effluent stream is recycled and introduced to the reactor with the feedstream. Since efficiency in such reactions never reaches 100 percent, the effluent stream always contains some carbon dioxide. In a reactor in which the effluent stream is recycled to the reactor, therefore, the feedstream always contains some carbon dioxide which, in combination with sodium and certain efficiency-enhancing compounds, results in losses of activity and stability of the catalyst and system. Commonly, the carbon dioxide is removed by a scrubbing device, such as a Benfield scrubber, placed in the effluent stream between the effluent outlet and the reactor inlet. Such devices require, however, a substantial capital expenditure and additional plant space. The catalyst and process of the present invention diminish the deactivating and life-shortening effects of CO.sub.2.